In addition to conventional horizontal beam control, 3D MIMO (Third Dimensional Multiple-Input Multiple-Output) for performing vertical beam control is being presently discussed. The 3D MIMO is sometimes classified into elevation beamforming (BF) and full dimension (FD)-MIMO depending on the number of antenna ports. As illustrated in FIG. 1, the 3D MIMO with up to eight antenna ports is referred to as the elevation beamforming, and the 3D MIMO with more antenna ports is referred to as the full dimension-MIMO or massive MIMO. In the full dimension-MIMO, as illustrated, not only two dimensional planar antennas having a large number of antenna ports but also three dimensional antennas, such as cylindrically arranged antennas and antennas arranged on surfaces of a cube, are used.
In the 3D MIMO where a base station having such multi-dimensional antennas controls beams in the two directions, that is, the horizontal direction and the vertical direction, active antenna system (AAS) based operations, where calibration is performed to form accurate beams in the vertical direction in consideration of impacts on inter-cell interference, are assumed. In Release 13 of LTE (Long Term Evolution) specifications, it is assumed that up to 64 transceiver units (TXRUs) are used, and these transceiver units are controlled under the active antenna system.
Effects of the 3D MIMO are as follows. First, a higher beam forming gain can be achieved by implementing the vertical beam control (precoding) in addition to the conventional horizontal direction. For example, as illustrated in FIG. 2, beams can be blown up toward user equipments in tall buildings. Also, the higher beam forming gain can be achieved with a larger number of antennas. For example, sharpened beams make it possible to transmit radio signals to targeted user equipment at higher transmission power as well as to reduce interference power from other beams. Furthermore, by using a huge amount of antenna elements, a transmission diversity gain can be achieved, and interference control and traffic offloading can be implemented with flexible beam control.
Antennas for use in the 3D MIMO typically have an arrangement as illustrated in FIG. 3. Specifically, V×H antenna elements are grouped into multiple subarrays. The illustrated subarrays are composed of vertically arranged antenna elements, but are not limited to it, and may be composed of horizontally or two-dimensionally or three-dimensionally arranged antenna elements. Also, the subarrays may not be necessarily composed of successive antenna elements. In general, the number of subarrays is the same as the number of TXRUs but is not necessarily limited to it. In cases where a single subarray is composed of one antenna element (K=1), the best transmission characteristics can be achieved, but more TXRUs are needed, which may increase workload of an associated baseband (BB) processing unit. Here, fixed tilt may be sometimes applied to antenna elements in the subarrays. Also, in the illustrated 3D MIMO antennas, single polarization antennas are used, but are not limited to it, and orthogonal polarization antennas may be used.
In the full dimension-MIMO or the massive MIMO, beam tracking error has a significant impact due to sharpened beams, which may result in holes in coverage. For this reason, appropriate beam forming is important, and various beam forming schemes are discussed. In other words, in the 3D MIMO, it has to be defined how a base station transmits reference signals for channel state measurement from multiple antenna ports and how user equipment feeds the measured channel state back.
See 3GPP TR 37.840 V12.1.0 (2013-12) and 3GPP TS 36.213 V12.2.0 (2014-06) for further details, for example.